The present invention relates generally to an image projection apparatus, and more particularly to a three-dimensional image displaying apparatus capable of displaying three-dimensional images on a screen.
With the advance of information transmission in the form of images, there has been a strong demand in recent years to display images on a large-size screen, and an image projection apparatus have been developed to meet such a demand. The image projection apparatus utilizes, for example, three cathode-ray tubes for producing three primary-color images and includes an optical system for projecting the primary-color images at an enlarged scale onto the screen so that an enlarged image is produced in color by combining the three primary-color images on the screen. In this image projection apparatus, the brightness of the displayed image is determined by the brightness of the image displayed on each cathode-ray tube, and it is difficult to obtain a bright image on a large screen. The prior art image projection apparatus, therefore, has a drawback in that the illumination in the room should be turned down for viewing.
Further, in a conventional three-dimensional image displaying apparatus, special eyeglasses have been required to view displayed images. According to one system, polarizers having directions of polarization displaced 90 degrees from each other are placed in front of two image display tubes for left and right eyes, the two images are combined with a half-silvered mirror, and the viewer views the combined image through eyeglasses having mutually orthogonal directions of polarization. In another system, one image display tube alternately displays images for left and right eyes in successive fields, and shutters are provided for alternately shielding the left and right eyes in synchronization with the alternate display of images. Still another system includes one image display tube for displaying the red and blue components of a color image and one for displaying the green component of the image, and eyeglasses are provided having a filter passing red and blue light for the left lens and a filter passing green light for the right lens. These three-dimensional television systems which require special eyeglasses are disadvantageous in that displayed images suffer from appreciable flickering and the viewer may suffer from eye fatigue after viewing such a display over a long period of time.
FIGS. 1 and 2 of the accompanying drawings illustrate a three-dimensional image displaying apparatus which has been proposed to dispense with special eyeglasses. The illustrated system has been described by Takanori Ohkoshi, Three-Dimensional Image Engineering, Japan: Sangyo Tosho Co., Ltd., 1972, pp. 217, 218. In FIG. 1, images displayed by an image tube 5 for the left eye and an image tube 6 for the right eye are combined by a half-silvered mirror 2, and the combined image is viewed from a viewing point 1. A double-layer slit plate as shown in FIG. 2A, or a combining of a slit plate and a lenticular lens as shown in FIG. 2B, is placed in each of positions 3 and 4. With this arrangement, the right eye views an image on the tube 5 and the left eye views an image on the tube 6. Further in FIGS. 2A and 2B, reference numerals 7 and 10 designate shield plates composed of vertical stripes, 8 indicates an image display surface, and 9 a lenticular lens. The construction of FIG. 2A is not preferable since images are divided into vertical stripes. The arrangement of FIG. 2B is somewhat better as it does not suffer from such problems. Both systems, however, have a drawback that since images are combined by a half-silvered mirror 2, the image position is far to the rear.
In still another known three-dimensional image displaying apparatus, the slit plate 7 shown in FIG. 2A or the lenticular lens 9 shown in FIG. 2B is placed immediately in front of a reflecting screen, and an image is projected onto the screen by two or more projectors. With two projectors, however, the range in which the images can be observed is small. One known screen capable of transmitting light therethrough has two lenticular lenses disposed back to back with a diffusion surface interposed therebetween. Such a screen is disclosed by Gen Yanagisawa, "History of Three-Dimensional Television and Its Various Systems", Graphics and Images, Spring 1982, p. 86; see especially FIGS. 21-a and 21-b. With this system, an image is projected from a projector onto a diffusion surface and the transmitted light is viewed. The difficulty with this system is that the range in which three-dimensional images can be viewed is small. Widening the viewing range requires many projectors and results in a system of large physical size.
There is known a still further system, disclosed in Examined Japanese Patent Publication No. 57-11550, in which a special three-dimensional television picture tube is employed for displaying three-dimensional images. The picture tube required is, however, of a special and complex construction.